(1) Field of the Invention
The present invention relates to a switch circuit which switches signals in a mobile communication device or the like and a technique used for a radio frequency power amplifier, and relates more specifically to a semiconductor device, a radio frequency circuit, and a radio frequency power amplifier.
(2) Description of the Related Art
Recently, cellular phones have been further equipped with multiband, multimode compatibility, with a demand growing for higher performance (lower loss) of a radio frequency switch using a field-effect transistor (FET). However, the radio frequency switch using the FET has a disadvantage of degrading isolation (inter-element isolation) characteristics at the time of supplying a large amount of power, and a technique of connecting plural FETs in series is used to improve this status. In this context, a switch circuit whose isolation characteristics are enhanced through multigate configuration has been proposed (for example, see Patent Reference 1: Japanese Patent No. 4202852).
FIG. 1 is a diagram showing a configuration of a switch circuit described in Patent Reference 1.
A conventional switch circuit shown in FIG. 1 includes: a switch transistor Q1 connected between a transmission-side terminal TX and a common terminal COM, and a switch transistor Q2 connected between the common terminal COM and a reception-side terminal Rx. Here, the transmission-side terminal Tx is connected to an output terminal of a power amplifier provided outside the diagram, the common terminal COM is connected to an antenna provided outside the diagram, and the reception-side terminal RX is connected to an input terminal of the receiving system circuit provided outside the diagram.
Depression-type HEMTs are used for the switch transistors Q1 and Q2, and each of the depression-type HEMTs is configured as a triple-gate element which includes three gate electrodes each corresponding to one channel.
This triple-gate structure allows the channel to be shorter due to absence of a region where source and drain electrode are supposed to be formed between adjacent gate regions, and also allows reducing on-resistance Ron due to absence of source and drain resistors, thus allowing reduction of insertion loss.
However, in this structure, when turning off the switch transistors Q1 and Q2 included in the conventional switch circuit, depletion layer capacitors directly beneath the multiple gates are connected in series. This does not allow reducing impedance of an off-channel, thus causing a problem of insufficient isolation between both ends of the off-channel.
In addition, when using, in a load circuit of a multiband, multimode power amplifier, the conventional switch circuit as a transmission line selection circuit which selects a transmission line according to each band or mode, the conventional switch circuit is not capable of converting the load impedance, thus causing another problem of degradation of efficiency characteristics of the power amplifier. Therefore, it can be said that the conventional switch circuit is not appropriate for multiband, multimode performance.